To extract petroleum fluids from a reservoir in an earth formation, wells are drilled into the earth formations. The development of drilling techniques has now evolved into the possibility of drilling wells in all directions to extract as much as possible out of a reservoir. A well may for instance comprise a mainly vertical section and at least one section which deviate from this vertical direction, possibly a mainly horizontal section. These sections of the well which deviate from a mainly vertical direction tend to become longer, and may extend for several thousand meters into a formation. The depth of the wells is also increasing and in addition the water depths for drilling wells are also increasing.
Drilling is normally performed by inserting a drilling bit on the end of a drill string into the well. The weight of the drill string is proportional with the length of the drill string. When drilling at large water depths the depth of the water also influences the pressure conditions in the well and the formation as such and adds to the weight of the drill string. During drilling one does not want formation fluid to penetrate the drilled well, so the pressure exerted by the drilling equipment on the formation should be higher than the formation pore pressure. With drilling equipment one should also understand the fluid added between the drill string and the unlined formation wall. With this one also has control of the well during drilling and will therefore prevent blow outs. At the same time there is also a need to limit the amount of drilling fluid that penetrate the unlined formation wall, and also a need to prevent fracturing the side wall of the drilled bore before production should start. This gives that the pressure exerted by the drilling equipment must not exceed a fracturing pressure of the formation. The formation pressure is also influenced by the hydrostatic pressure, and at larger water depths this also increases. When the pressure exerted by the drilling equipment moves towards the boundaries, the fracturing pressure or the formation pore pressure, the well needs to be provided with casings or liners before one may drill further in the well. This would often mean pulling the drilling equipment out of the well, and providing new sections of casing or liners in the well before one may continue with the drilling. There is therefore a general need to develop methods for performing drilling where the drilling for a longer period may be performed in the allowed pressure range, between the formation pore pressure and the formation fracturing pressure.
Another element is when the well deviates from a vertical direction at least a part of the drill string will due to gravity forces also tend to come in contact with the wall of the bore hole. For a horizontal section the drill pipe will tend to rest on the relative lower part of the borehole wall. This contact between the drill string and the borehole wall will create friction as the drill string is moved further into the well during drilling or when it is moved out or into the well.
As wells are drilled at greater water depths and further into the ground and deviated well becomes longer, the weight of the drill string and friction forces increases. There will naturally be a limit on how much weight and friction forces the equipment for performing the drilling may take and this will limit the reach of a conventional drilling string.
In U.S. Pat. No. 5,060,737 there is described a drilling system for deviated drilling. There is in this publication described several ways of providing advancement of the drill string in the well bore. In US2004/0104052, WO 2004/018828, WO2006/014417 and US2008/0073123 there are described different methods for performing drilling with a dual bore pipe. There is in U.S. Pat. No. 5,964,294 described a tool for performing a down hole function in a horizontal or highly deviated well.